It is well known to construct motor vehicle body doors, hoods, fenders, tailgates, trunk and deck lids by stamping an outer sheet metal panel and separately stamping an inner sheet metal reinforcing panel with an outer periphery generally matching that of the outer panel, and then joining the two panels together by hemming a flange-over, the periphery of the outer panel over an adjacent edge of the inner panel to thereby secure the panels together. The inner and outer panels are individually stamped to their desired size and shape, with the outer panel being slightly larger than the inner panel to provide a border flange portion along the edge of the outer panel having an upstanding lip which can be folded over the peripheral edge of the inner panel to define the hem flange which connects the two panels.
It has been recognized in the prior art that this hem flanging together of the panels may not be sufficient to prevent the inner panel from sliding relative to the outer panel. Accordingly, it has been known to employ auxiliary attachment techniques to lock the panels against relative movement. For example, it has been known to apply an arc, mig, or fusion weld to tack the hemmed-over edge of the outer panel to the inner panel. It is also known to spot weld the two panels together at the flange. In still other situations, induction heat has been used to cure an adhesive previously applied between the panels at the area of the flange. In each of these cases, the operation require additional equipment and inevitably damages or distorts the assembled panels, which in turn requires refinishing of the metal to correct the imperfections which will become visible i.e., "read through", when the outer panel is painted.
It has also been known to punch holes in the edge of the inner panel during the progressive die stamping operations typically employed in making the inner panel. Then in the downstream hemming station corresponding projections are provided on the face of the hem flanging punch so that the punch which flanges the outer panel over the inner panel will also press and coin the flange into the holes in the inner panel. Although this mechanical coined interlock technique eliminates the additional investment and labor of a welding operation, it nonetheless has been found to distort the outer panel because the punching burrs or upset edge on the inner panel outboard side around each punched hole in the inner panel causes read through on the exterior or outboard surface of the outer panel.
Another prior approach to overcoming the problems of interlocking the hemmed together flanges of the inner and outer panels is that disclosed in U.S. Pat. No. 5,237,734, issued Aug. 24, 1993, the disclosure of which is incorporated herein by reference. In the '734 patent a plurality of raised beads are formed at spaced intervals along the length of the edge of the inner panel. The inner panel is placed inside the outer panel with the raised beads facing away from the outer panel. Then the edge of the outer panels is hemmed over the edge of the inner panel by a hemming punch having a plurality of slots therein located to register with the raised beads of the inner panel so that the outer panel is coined over, and a complementarily shaped raised bead nested onto, the inner panel raised beads to thereby interlock the inner and outer panels together against relative movement. The inner panel raised beads are stated to preferably have a height at least equal to the thickness of the inner panel, and are preferably elongated in shape with the elongation extending in the direction either parallel or perpendicular to the edge of the panel. The slots in the hemming die are stated to preferably have a length longer than the mating raised bead of the inner panel to allow at least one millimeter of clearance between the panels at each end of the beads.
However, the '734 bead-on-bead method does not provide the same mechanical interlock strength as the aforementioned punched hole method in which the outer panel flange is pressed coined into the hole of the inner panel as shown in FIG. 4 of the '734 patent. Additionally, the '734 method necessarily results in an undesirable overall increase in the thickness dimension of the hemmed flange as compared to the prior punched hole method.
Another problem common to both such prior art methods has been found to arise as a result of the interlock holes or the raised interlock beads being initially formed in the edge of the inner panel concurrently with punching principal locating point (PLP) holes. These PLP holes are used for properly reorienting the inner panel alone and also often marrying to the outer panel in successive downstream fixturing and processing steps involved in producing a hem flange interlock of the panels. Typically the PLP locating holes are punched through the inner panel by a piercing tool in the same stage of progressive die forming in which the interlock holes or raised beads are formed. The raised beads, like the interlock holes, can and do shift a small amount in their location relative to that of the PLP holes of the inner panel due to minute changes in the inner panel three-dimensional contour and configuration during the subsequent downstream processing and transferring steps involved prior to and in the hemming station(s). Such dimensional changes have been found to occur primarily as a result of various hardware components being sub-assembled to the inner panel before it is married to the outer panel. Such components typically are fastened by means of spot welding.
For example, after being finished stamped, the inner panel for a vehicle body front or rear side door goes through a pre-assembly procedure which involves assembling and welding various components fixedly onto the inner panel, such as a hinge reinforcement, an impact or crash bar, possibly also a window belt reinforcement member, a lock striker reinforcement piece, etc. The stresses introduced in this subassembly process become "locked in" and thereby introduce a small but measurable change in the location of the interlock holes or raised beads relative to the PLP holes. Moreover, shifts in location are not necessarily uniformly predictable from part to part.
Accordingly, in the downstream final hemming operation, when it is desired to deform the metal of the folded down flange of the outer panel either into an interlock hole or over a raised dimple, the location of these potential interlock points on the inner panel may have shifted relative to the design orientation of the forming tooling provided in the final hemming steel of the hemming machine or in a separate staking station downstream therefrom. De-registry of the outer panel metal, as worked by the forming tool, with its intended location relative to the interlock hole or raised bead or dimple thus can and often does result, thereby causing an imperfect or defective interlock joint at such de-registered locations in the hemmed inner and outer panels.
Preferably in the downstream hemming process stage a hemming press of the improved type disclosed and claimed in U.S. Pat. No. 5,457,981, issued Oct. 17, 1995 and assigned to Western Atlas, Inc. of Warren, Mich., assignee of record herein, (which is incorporated herein by reference) is employed to perform in one station both a prehemming operation that bends the lip edge of the outer panel to an acute included angle with respect to the outer panel, and then a final hemming operation to completely bend the prehemmed edge of the inner outer panel over the peripheral edge of the reinforcing panel to thereby secure and attach the panels together as a unitary structural member for assembly on a vehicle. However, in some cases the outer panel, due to its three-dimensional cambered geometry, cannot be stamped so that the lip flange around the outer periphery of the outer panel remains at a right angle to the adjacent portion of the outer panel. In such situations, it is preferred to use a separate prehemming station employing the improved prehemming machine as disclosed and claimed in U.S. Pat. No. 5,507,165, issued Apr. 16, 1996, to William R. Hartley (inventor herein) and also assigned of record to Western Atlas, Inc. (the disclosure of which is also incorporated herein by reference). It of course would be highly desirable from an efficiency standpoint to combine the interlock cold forming operation with the final hemming operation, such as in the manner suggested in the aforementioned '734 patent and particularly by employing the improved dual pre-and-final hemming press machine of the aforementioned '981 patent. However, upstream processing-induced shifting of interlock holes or dimples has hitherto posed a serious obstacle to achieving successful registry of staking tooling, if provided in the hemming steel, with the interlock holes.
In any event, regardless of the mass production operations and precision equipment hitherto utilized in constructing and assembling the inner and outer panels, the problem of de-registry of the panel interlock holes or dimples with the principal locating points of the inner panel still remain. This condition results in the possibility of panels loosening from each other, becoming skewed with respect to each other, resulting in a finished hemmed assembly of lesser quality and poor structural integrity. An assembly with these characteristics may have to be repaired or scrapped, thereby increasing production cost and lowering profits. Even worse, an ill-assembled structural member with these flaws when incorporated into an assembled vehicle may fit poorly and affect perceived quality by prospective purchasers, thereby reducing vehicle sales and profits. An assembled defective structural member may further lose integrity as the vehicle is subjected to road vibration during use and possibly require replacement and thus negatively impact an owners future vehicle purchasing decision.